The present invention relates to a magnetic recording medium especially adapted for use with a rotary head type video tape recorder.
In the fabrication of thin ferromagnetic film type recording media, an vacuum evaporation, sputtering or ion-plating process is employed so that iron, cobalt, nickel or their alloy may be deposited over a base of a high-weight compound film such as polyester or polyimide film. Such thin ferromagnetic film type magnetic recording media are by far advantageous over the prior art coating type magnetic recording media consisting of a base and a thin magnetic layer in that the recording density can be remarkably increased. However, in order to attain a higher degree of recording density, the surfaces of a magnetic recording medium must be made as flat as possible so that "spacing loss" may be reduced to a minimum. But if the surface is extremely smooth or flattened, it makes intimate contact with a magnetic head so that the smooth transportation of the magnetic tape past the head is hindered. With the rotary head type video tape recorder system which has been increasingly popular in the market, the use of thin ferromagnetic film type magnetic recording media is almost imperative in order to improve the recording denisty. Such magnetic recording media or tapes must satisfy various demands in practice. For instance, they must make a suitable contact with rotary heads so as to avoid excessive wear or abrasion of the heads, thereby preventing the clogging of gaps thereof. Furthermore, even when they are in contact with a rotary head cylinder, tape guide posts, a stationary audio head and so on, they must move past them smoothly. In other words, they must have a low coefficient of friction and exhibit a high degree of resistance against wear. The magnetic layer of a thin ferromagnetic film type magnetic recording tape is extremely thin and is of the order of from 0.1 to 0.5 .mu.m so that the above-described properties or characteristics are all dependent upon the surfaces of the plastic film which is the base of the magnetic recording medium. Therefore, various proposals have been made in order to provide satisfactory surface textures of the bases. For instance, such proposals were disclosed in Japanese Laid Open Patent Application Nos. 116115/1978, 128683/1978, 94574/1979, 10455/1981 and 16937/1981. These proposals are common in that they impart to the surface relatively uniform and minute roughness. For instance, the surface is formed with rumple- or worm-like nodules or with grain-like nodules. The common object of such proposals is, therefore, to maintain suitable contact with the magnetic heads, thereby improving the transportability; that is, the smooth travel of the magnetic tape past the heads and the like. In the case of the rotary head assembly of a video tape recorder, the width of contact between a magnetic tape and a head is extremely narrow and is of the order of less than hundreds of microns. Furthermore, the relative velocity between the magnetic tape and the head is extremely high and is of the order of a few meters per second, whereas in the tape transport system, the magnetic tape is transported at a relatively slow velocity of the order of a few centimeters per second and makes contact with various component parts in the transport system with relatively large areas. Therefore, the surface texture design becomes a compromise between the ability of tape to move smoothly past the rotary head assembly and its associated parts and the ability of being smoothly transported through the tape transport system. In addition, the surface texture must be so designed as to have some abrasive action on the rotary heads in order to prevent clogging of the gaps thereof.